don't worry about replacing the rear o2 unles you are gettign a cel.
it serves absolutely zero purpose in fuel efficiency. its only there for monitering catalytic converter function.
as for the front.
replace it with a oem ntk sensor.
nothing else comes close in precision and durability.
i reccomend a plug and play sensor, makes it a simple 10 minute affair.
if you are savvy and you want to save some cash,
get a universal ntk, and wire it in.
you could get a bosch, but they don't last.
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